It is allowed to move the same piece as the opponent moved before, possibly (in case of a piece that is not a pawn) undoing the move. It is also allowed to move a pinned piece of the opponent such that his king goes into check. (Pritchard does not state if a king may be moved into check - one would assume that one is also allowed to move the king of the opponent into check.)
Last modified: Sunday, April 1, 2012